Superlattice Structure

ABSTRACT

A superlattice layer including a plurality of periods, each of which is formed from a plurality of sub-layers is provided. Each sub-layer comprises a different composition than the adjacent sub-layer(s) and comprises a polarization that is opposite a polarization of the adjacent sub-layer(s). In this manner, the polarizations of the respective adjacent sub-layers compensate for one another. Furthermore, the superlattice layer can be configured to be at least partially transparent to radiation, such as ultraviolet radiation.

REFERENCE TO RELATED APPLICATIONS

The current application claims the benefit of U.S. ProvisionalApplication No. 61/610,636, which was filed on 14 Mar. 2012 and U.S.Provisional Application No. 61/768,799, which was filed on 25 Feb. 2013,both of which are hereby incorporated by reference. Additionally, thecurrent application is a continuation-in-part of co-pending U.S. patentapplication Ser. No. 13/162,895, titled “Superlattice Structure,” whichwas filed on 17 Jun. 2011, which is a continuation-in-part of co-pendingU.S. patent application Ser. No. 12/987,102, titled “SuperlatticeStructure,” which was filed on 8 Jan. 2011, and which claims the benefitof co-pending U.S. Provisional Application No. 61/293,614, titled“Superlattice Structures and Devices,” which was filed on 8 Jan. 2010,all of which are hereby incorporated by reference.

GOVERNMENT LICENSE RIGHTS

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of the Grant No.IIP-0839492 awarded by the National Science Foundation.

TECHNICAL FIELD

The disclosure relates generally to semiconductor devices, and moreparticularly, to a superlattice structure configured to reducepolarization effects of the semiconductor materials forming the devices.

BACKGROUND ART

In nitride based semiconductor materials and devices, including visibleand ultraviolet (UV) light emitting diodes (LEDs), polarization effectsplay a dominant role causing strong built-in fields and spatialseparation of electrons and holes. These polarization effects cannegatively impact the performance of nitride-based visible andultraviolet light emitting diodes. For example, FIGS. 1A-1C showillustrative band diagrams of a positive-intrinsic-negative (p-i-n)quantum well structure according to the prior art. In particular, FIG.1A shows a band diagram of the structure without external bias andillumination; FIG. 1B shows a band diagram of the structure with thep-i-n field compensated by external bias; and FIG. 1C shows a banddiagram of the structure with the total electric field compensated byexternal bias and intense optical excitation.

Polarization effects were evaluated for illustrative aluminum indiumgallium nitride-based (Al_(x)In_(y)Ga_(1-x-y)N-based) multiple quantumwell (MQW) structures. The MQW structures comprise an Al molar fractionin the quantum wells and barrier layers close to 20% and 40%,respectively, and In content in both the quantum wells and barriers ofapproximately 2% and 1%, respectively. The MQW structures comprise atotal of three wells, each of which is two to four nanometers thick,separated by four five nanometer thick barriers.

Calculations indicated that the barriers and wells undergo tensions of0.815% and 0.314%, respectively. These tensions correspond topiezoelectric charges at interfaces induced by this mismatch of −0.0484coulombs per meter squared (C/m²) for the well and −0.0134 C/m² for thebarrier. The polarization charge was calculated as −0.041 C/m² and−0.049 C/m² for the wells and barriers, respectively. The total electricfield in the well for an alternating sequence of barriers and wells wasfound to be 1.2 Megavolts per centimeter (MV/cm). About fifty percent ofthe field was due to piezoelectric effect and the remaining fiftypercent was caused by spontaneous polarization, both having the samedirection. This corresponds to a 0.12 eV band bending in a one nanometerwide quantum well. Such band bending precludes using wide quantum wellsin deep UV LEDs, which decreases the overall LED efficiency by limitingthe MQW design optimization to very narrow (i.e., one to two nanometerthick) quantum wells.

SUMMARY OF THE INVENTION

Aspects of the invention provide a superlattice layer including aplurality of periods, each of which is formed from a plurality ofsub-layers. Each sub-layer comprises a different composition than theadjacent sub-layer(s) and comprises a polarization that is opposite apolarization of the adjacent sub-layer(s). In this manner, thepolarizations of the respective adjacent sub-layers compensate for oneanother. The superlattice layer can be incorporated in various types ofdevices, and can allow for, for example, utilization of much widerquantum wells by avoiding the detrimental confined Stark effect, whichprevents efficient radiative recombination. Furthermore, thesuperlattice layer can be configured to be at least partiallytransparent to radiation, such as ultraviolet radiation.

A first aspect of the invention provides a structure comprising: a firstlayer; and a superlattice layer having a first side adjacent to thefirst layer, the superlattice layer including a plurality of periods,each of the plurality of periods including: a first sub-layer having afirst composition and a first polarization; and a second sub-layeradjacent to the first sub-layer, the second sub-layer having a secondcomposition distinct from the first composition and a secondpolarization opposite the first polarization.

A second aspect of the invention provides a method comprising: creatinga structure design for a device, the structure design including a firstlayer and a superlattice layer having a first side adjacent to the firstlayer, the superlattice layer comprising a plurality of periods, thecreating the structure design including: selecting a first compositionhaving a first polarization for a first sub-layer of each of theplurality of periods; and selecting a second composition having a secondpolarization for a second sub-layer of each of the plurality of periods,wherein the second sub-layer is adjacent to the first sub-layer, andwherein the second composition is distinct from the first compositionand the second polarization is opposite the first polarization.

A third aspect of the invention provides a group III nitride-baseddevice comprising: a p-type contact comprising: a first p-type layer;and a p-type superlattice layer including a plurality of periods, eachof the plurality of periods including: a first sub-layer having a firstgroup III nitride-based composition and a first polarization; and asecond sub-layer adjacent to the first sub-layer, the second sub-layerhaving a second group III nitride-based composition distinct from thefirst composition and a second polarization opposite the firstpolarization, wherein the first polarization and the second polarizationcomprise at least one of: a strain-induced polarization or a spontaneouspolarization.

A fourth aspect of the invention provides a structure comprising: afirst layer; and a superlattice layer having a first side adjacent tothe first layer, the superlattice layer including a plurality ofperiods, each of the plurality of periods including: a first sub-layerhaving a first group III nitride composition and a first polarization,wherein the first group III nitride composition is selected such thatthe first sub-layer has a transparency of at least a target transparencyto ultraviolet radiation of a target wavelength; and a second sub-layeradjacent to the first sub-layer, the second sub-layer having a secondgroup III nitride composition distinct from the first group III nitridecomposition and a second polarization opposite the first polarization.

A fifth aspect of the invention provides a method comprising: creating astructure design for a device, the structure design including a firstlayer and a superlattice layer having a first side adjacent to the firstlayer, the superlattice layer comprising a plurality of periods, thecreating the structure design including: selecting a first group IIInitride composition having a first polarization for a first sub-layer ofeach of the plurality of periods, wherein the first group III nitridecomposition is selected such that the first sub-layer has a transparencyof at least a target transparency to ultraviolet radiation of a targetwavelength; and selecting a second group III nitride composition havinga second polarization for a second sub-layer of each of the plurality ofperiods, wherein the second sub-layer is adjacent to the firstsub-layer, and wherein the second group III nitride composition isdistinct from the first group III nitride composition and the secondpolarization is opposite the first polarization.

A sixth aspect of the invention provides a group III nitride-baseddevice comprising: a p-type contact comprising: a first p-type layer;and a p-type superlattice layer including a plurality of periods, eachof the plurality of periods including: a first sub-layer having a firstgroup III nitride-based composition and a first polarization, whereinthe first group III nitride-based composition is selected such that thefirst sub-layer has a transparency of at least a target transparency toultraviolet radiation of a target wavelength; and a second sub-layeradjacent to the first sub-layer, the second sub-layer having a secondgroup III nitride-based composition distinct from the first compositionand a second polarization opposite the first polarization, wherein thefirst polarization and the second polarization comprise at least one of:a strain-induced polarization or a spontaneous polarization.

The illustrative aspects of the invention are designed to solve one ormore of the problems herein described and/or one or more other problemsnot discussed.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the disclosure will be more readilyunderstood from the following detailed description of the variousaspects of the invention taken in conjunction with the accompanyingdrawings that depict various aspects of the invention.

FIGS. 1A-1C show illustrative band diagrams of a p-i-n quantum wellstructure according to the prior art.

FIGS. 2A and 2B show illustrative structures according to the prior artand an embodiment, respectively.

FIG. 3 shows a conduction band diagram comparing a conduction bandprofile for a conventional quantum well and a conduction band profilefor a quantum well according to an embodiment.

FIG. 4 shows another illustrative structure according to an embodiment.

FIG. 5 shows a chart of a calculated electric field at a heterointerfacebetween gallium nitride (GaN) and aluminum indium nitride (AlInN) as afunction of the indium molar fraction in the AlInN according to anembodiment.

FIG. 6 shows an illustrative light emitting device structure accordingto an embodiment.

FIG. 7 shows a dependence of the absorption coefficient on thewavelength for various aluminum molar fractions (x) of anAl_(x)Ga_(1-x)N alloy according to an embodiment.

FIG. 8 shows an illustrative chart for selecting an aluminum content ofan AlGaN alloy to maintain a target transparency for a correspondingemitted wavelength according to an embodiment.

FIG. 9 shows an illustrative lattice configuration of a gallium nitridelayer including domain inversion according to an embodiment.

FIG. 10 shows an illustrative flow diagram for fabricating a circuitaccording to an embodiment.

It is noted that the drawings may not be to scale. The drawings areintended to depict only typical aspects of the invention, and thereforeshould not be considered as limiting the scope of the invention. In thedrawings, like numbering represents like elements between the drawings.

DETAILED DESCRIPTION OF THE INVENTION

As indicated above, aspects of the invention provide a superlatticelayer including a plurality of periods, each of which is formed from aplurality of sub-layers. Each sub-layer comprises a differentcomposition than the adjacent sub-layer(s) and comprises a polarizationthat is opposite a polarization of the adjacent sub-layer(s). In thismanner, the polarizations of the respective adjacent sub-layerscompensate for one another. The superlattice layer can be incorporatedin various types of devices, and can allow for, for example, utilizationof much wider quantum wells by avoiding the detrimental confined Starkeffect, which prevents efficient radiative recombination. Furthermore,the superlattice layer can be configured to be at least partiallytransparent to radiation, such as ultraviolet radiation. As used herein,unless otherwise noted, the term “set” means one or more (i.e., at leastone) and the phrase “any solution” means any now known or laterdeveloped solution.

Turning to the drawings, FIGS. 2A and 2B show illustrative structures 2,10 according to the prior art and an embodiment, respectively. Asillustrated in FIG. 2A, structure 2 includes a superlattice layer 4,which includes a plurality of repeating sub-layers 6A-6C. Each sub-layer6A-6C can be separated from another sub-layer by a second set ofsub-layers 8A-8B in the superlattice layer 4. Superlattice structure 4can be configured to perform any type of function as part of a deviceincorporating structure 2. For example, sub-layers 6A-6C can comprise aset of quantum wells and sub-layers 8A-8B can comprise a set ofbarriers. In this case, superlattice layer 4 can comprise a multiplequantum well structure.

As shown in FIG. 2B, an embodiment of the invention provides a structure10 including a superlattice layer 12 that is configured, for example, toreduce polarization effects. In particular, the superlattice layer 12includes multiple periods 14A-14C, each of which includes two or moresub-layers 16, 18 having different compositions. Adjacent sub-layers 16,18 in each period 14A-14C are configured to have polarizations (e.g.,built-in electric fields) that at least partially cancel one another.For example, sub-layer 16 can comprise a spontaneous polarization havingan opposite sign as a spontaneous polarization of sub-layer 18.Similarly, sub-layer 16 can comprise a strain-induced polarizationhaving an opposite sign of a strain-induced polarization of sub-layer18. Still further, one type of polarization in sub-layer 16 can have anopposite sign of another type of polarization in sub-layer 18, therebyreducing the net polarization present due to a combination of multipletypes of polarizations (e.g., spontaneous and strain-induced).

In an embodiment, sub-layer 16 can comprise a positive or negativespontaneous polarization, while sub-layer 18 comprises the other of thepositive or negative spontaneous polarization. In a more particularembodiment, the absolute values of the spontaneous polarizations ofsub-layers 16, 18 are substantially equal, so that the net spontaneouspolarization for the period 14A-14C is close to zero. In anotherembodiment, sub-layer 16 can comprise a strain-induced (e.g.,piezoelectric) polarization due to stretching or compression, whilesub-layer 18 comprises a strain-induced polarization due to the other ofstretching or compression. In a more particular embodiment, the absolutevalues of the strain-induced polarizations of sub-layers 16, 18 aresubstantially equal, so that the net strain-induced polarization for theperiod 14A-14C is close to zero. It is understood that the respectivespontaneous and/or strain-induced polarizations of sub-layers 16, 18 canbe configured to only partially reduce the net spontaneous and/orstrain-induced polarization for the period 14A-14C.

In still another embodiment, the spontaneous and/or strain-inducedpolarization of one sub-layer 16, 18 is configured to at least partiallycompensate the other of the spontaneous and/or strain-inducedpolarization of the other sub-layer 16, 18. For example, sub-layer 16can comprise a spontaneous polarization of a first sign, and sub-layer18 can comprise a strain-induced polarization of the opposite sign. Inthis case, the net polarization for the period 14A-14C will be reduceddue to the two types of polarizations of the sub-layers 16, 18compensating one another.

The various periods 14A-14C in superlattice layer 12 can be separatedfrom one another by a set of additional sub-layers 20A-20B. In anembodiment sub-layers 20A-20B comprise inactive layers having nopolarization. In another embodiment, each period 14A-14C comprises aquantum well, while each sub-layer 20A-20B comprises a barrier. In thiscase, superlattice layer 12 comprises a multiple quantum well structure.The periods (e.g., quantum wells) 14A-14C in superlattice layer 12 canbe wider than the conventional sub-layers (e.g., quantum wells) 6A-6C.For example, in an embodiment, the width of superlattice layer 12 can begreater than two nanometers. In a more particular embodiment, the widthof superlattice layer 12 is between approximately three nanometers andeight nanometers. In particular, periods 14A-14C will comprise a muchsmaller polarization field than that of a conventional sub-layer 6A-6Cof a similar width. As a result, the detrimental confined Stark effectis avoided, which separates electrons and holes within a quantum welland prevents efficient radiative recombination.

FIG. 3 shows a conduction band diagram comparing a conduction bandprofile 22 for a conventional quantum well 6A (FIG. 2A) and a conductionband profile 24 for a quantum well 14A (FIG. 2B) according to anembodiment. As illustrated, the conduction band profile 24 comprises amore shallow profile than that of the conduction band profile 22. As aresult, electrons in quantum well 14A can spread out within the quantumwell 14A more than the electrons in quantum well 6A, providing for amore efficient radiative recombination.

Returning to FIG. 2B, superlattice 12 can perform any function as partof a device formed using structure 10. To this extent, superlattice 12is located between a first layer 26 and a second layer 28 of thestructure 10. In an illustrative embodiment, first layer 26 and secondlayer 28 can be formed from two dissimilar materials (e.g., twodissimilar nitride based semiconductor materials), and superlattice 12can be graded in such a manner that it compensates (e.g., reduces)strain exerted by the dissimilar materials of layers 26, 28. Forexample, the lattice structure of each sub-layer 16, 18 of superlattice12 can gradually change from a lattice structure similar to first layer26 to a lattice structure similar to second layer 28.

While periods 14A-14C are each shown including two sub-layers 16, 18. Itis understood that each period 14A-14C can include any number ofsub-layers 16, 18. Similarly, while superlattice layer 12 is shownincluding three periods 14A-14C, it is understood that superlatticelayer 12 can include any number of two or more periods 14A-14C. Forexample, FIG. 4 shows another illustrative structure 30 according to anembodiment. Structure 30 includes a superlattice layer 32, whichcomprises four periods 34A-34D that are separated by three sub-layers36A-36C. Each period 34A-34D is formed by a set of six sub-layers ofalternating compositions and polarizations. To this extent, eachsub-layer of each period 34A-34D is immediately adjacent to one or twosub-layers having a different composition and an opposite polarization(e.g., spontaneous and/or strain-induced as described herein). In thismanner, the periods 34A-34D can be made even wider than the conventionalsub-layers 6A-6C of the prior art with smaller polarization fields thana conventional sub-layer 6A-6C of a similar thickness.

In an embodiment, structures 10 (FIG. 2B) and 30 (FIG. 4) can comprisenitride-based heterostructures. In a more specific embodiment, thestructures 10, 30 comprise group III nitride-based heterostructures. Inthis case, the periods 14A-14C, 34A-34D of each structure 10, 30,respectively, each can be formed of group III nitride materials. GroupIII nitride materials comprise one or more group III elements (e.g.,boron (B), aluminum (Al), gallium (Ga), and indium (In)) and nitrogen(N), such that B_(W)Al_(X)Ga_(Y)In_(Z)N, where 0≦W, X, Y, Z≦1, andW+X+Y+Z=1. Illustrative group III nitride materials include AlN, GaN,InN, BN, AlGaN, AlInN, AlBN, AlGaInN, AlGaBN, AlInBN, and AlGaInBN withany molar fraction of group III elements. In an even more specificembodiment, the sub-layers described herein are quaternary or ternarygroup III nitride sub-layers, such as such as AlInN, AlGaN, InGaN, orAlInGaN. For further strain and/or polarization reduction, one or moresub-layers 16, 18 forming each period 14A-14C, 34A-34D can be doped. Thesub-layers 16, 18 can be doped p-type or n-type. Furthermore, asub-layer 16, 18 can comprise a monolayer.

FIG. 5 shows a chart of a calculated electric field at a heterointerfacebetween gallium nitride (GaN) and aluminum indium nitride (AlInN) as afunction of the indium molar fraction in the AlInN according to anembodiment. As illustrated, the calculated electric field drops to zeroand goes negative as the indium molar fraction exceeds 0.7. In anillustrative embodiment, each sub-layer 16, 18 comprises AlInN withdiffering molar fractions of In. For example, sub-layer 16 can comprisean In molar fraction of approximately 0.65, which results in acalculated electric field of approximately 0.5 MV/cm, and sub-layer 18can comprise an In molar fraction of approximately 0.77, which resultsin a calculated electric field of approximately −0.5 MV/cm. In thismanner, the electric fields of both sub-layers 16, 18 can substantiallycancel one another.

The superlattices 12, 32 described herein can be implemented as part ofstructures 10, 30 utilized for various types of devices, e.g., which arefabricated using semiconductor materials where polarization effects playa role. A superlattice 12, 32 described herein can be utilized as, forexample, a multiple quantum well, an integral part of an ohmic and/orSchottky contact, a cladding layer, a buffer layer, a barrier layer,and/or the like, for the device. In an illustrative embodiment,structure 10 comprises a p-type contact including superlattice 12 and ametal layer 26 located thereon.

A structure 10, 30 described herein can be implemented as part of, forexample, a light emitting device, such as a light emitting diode (LED),a superluminescent diode, or a laser. The light emitting device cancomprise a visible light emitting device, an ultraviolet light emittingdevice, and/or the like. In this case, the light emitting device caninclude one or more superlattices as cladding layer(s), ohmiccontact(s), and/or the like. In a more particular embodiment, thesuperlattice is formed as part of an ohmic contact for an ultravioletlight emitting device where a top p-type contact layer (e.g., layer 26of FIG. 2B) of the ohmic contact, which is transparent to ultravioletradiation, is located directly on the superlattice 12, 32. In a stillmore particular embodiment, the top p-type contact layer comprisesAlInN.

It is understood that any combination of one or more layers (orsub-layers) in a structure 10, 30 can be configured to be at leastpartially transparent (e.g., semi-transparent or transparent) toradiation, such as ultraviolet radiation. As used herein, a layer is atleast partially transparent to ultraviolet radiation if it allows morethan approximately 0.001 percent of the ultraviolet radiation to passthere through. In a more particular embodiment, an at least partiallytransparent layer is configured to allow more than approximately fivepercent of the ultraviolet radiation to pass there through. In anembodiment, the at least partially transparent layer(s) are configuredto be at least partially transparent to ultraviolet radiation emitted bythe structure 10, 30. For example, the at least partially transparentlayer(s) can be configured to be at least partially transparent toultraviolet radiation in a range including the peak emission wavelengthof the structure 10, 30 and at least five nanometers above and/or belowthe peak emission wavelength.

The layer(s) at least partially transparent to the ultraviolet radiationcan be formed using any solution. For example, a transparent layer cancomprise a p-type layer formed of a group III nitride material describedherein. Illustrative at least partially transparent group-III nitridematerials include AlGaN, AlInGaN, boron-containing alloys (GaBN, AlBN,AlGaBN, AlInGaBN, InGaBN, and/or the like), and/or the like.Furthermore, the at least partial transparency of a layer can beachieved using any solution. For example, at least partial transparencycan be achieved in materials with bandgaps smaller than a photon energyof the ultraviolet radiation due to tunneling, thermionic transport viaimpurity states, and/or the like.

Similarly, it is understood that any combination of one or more layersin a structure 10, 30 can be configured to reflect ultravioletradiation. As used herein, a layer is reflective of ultravioletradiation when it reflects more than approximately five percent of theultraviolet radiation. In an embodiment, the reflective layer(s) areconfigured to reflect ultraviolet radiation emitted by the structure 10,30. For example, the reflective layer(s) can be configured to reflectultraviolet radiation in a range including the peak emission wavelengthof the structure 10, 30 and at least five nanometers above and/or belowthe peak emission wavelength.

The ultraviolet reflective layer(s) can be formed using any solution.For example, a reflective layer can comprise a metal coating formed ofAl, Rhodium (Rh), enhanced Al, enhanced Rh, Gold (Au), Aluminum SiliconMonoxide (AlSiO), Aluminum Magnesium Fluoride (AlMgF₂), and/or the like.Furthermore, the reflectivity of a layer can be achieved using anysolution. For example, reflectivity can be achieved by the formation ofa reflecting photonic crystal, a distributed Bragg reflector (DBR)structure, and/or the like.

The at least partially ultraviolet transparent and/or reflectivelayer(s) can comprise any of various layers of a structure 10, 30 basedon a desired operating configuration for the structure 10, 30. Forexample, a structure 10, 30 can include an at least partiallyultraviolet transparent contact. Such a contact can comprise, forexample, a p-type at least partially ultraviolet transparent layer 26(FIG. 2B) and an interlayer, such as an at least partially ultraviolettransparent superlattice 12 (FIG. 2B), for making a p-type ohmiccontact, Schottky contact, non-ohmic contact, and/or the like.Similarly, a structure 10, 30 can include an ultraviolet reflectingcontact, which is configured to reflect a desired amount of theultraviolet radiation generated by the structure 10, 30. Such areflecting contact also can include, for example, a p-type ultravioletreflective layer 26 and a superlattice 12 for making a p-type ohmiccontact, Schottky contact, non-ohmic contact, and/or the like.

A structure 10, 30 can include various other layers, which are at leastpartially ultraviolet transparent and/or ultraviolet reflective, such asa p-type superlattice 12, 32, an electron blocking layer located betweena superlattice 12, 32 and a multiple quantum well structure, and/or thelike. In each case, the at least partially ultraviolet transparentand/or ultraviolet reflective layer can be formed using any type ofmaterial. In an embodiment the at least partially ultraviolettransparent and/or ultraviolet reflective layer is formed using agroup-III nitride material, such as boron-containing layers.

FIG. 6 shows an illustrative light emitting device structure 40according to an embodiment. As illustrated, the device structure 40comprises an n-type contact layer 50 adjacent to a radiation generatingstructure 52. Radiation generating structure 52 can comprise any type ofstructure, such as a multiple quantum well structure, for generating anytype of radiation, such as ultraviolet light. Furthermore, devicestructure 40 includes a p-type contact layer 54 on an opposing side ofthe radiation generating structure 52 as the n-type contact layer 50.

The device structure 40 further includes a superlattice layer 12, whichcan be formed as described herein. Superlattice layer 12 is shownlocated on the same side of the radiation generating structure 52 as thep-type contact layer 54. In an embodiment, the superlattice layer 12 isat least partially transparent to radiation generated by radiationgenerating structure 52. It is understood that superlattice layer 12 isonly illustrative of the types of superlattices that can be included inthe device structure 40. For example, the device structure 40 couldinclude superlattice 32 and/or a variant of the superlattices shownherein.

The device structure 40 also can include an electron blocking layer 56,which can be located between the superlattice layer 12 and the radiationgenerating structure 52. In an embodiment, the electron blocking layer56 has a thickness in a range between approximately two andapproximately one hundred nanometers. The electron blocking layer 56 cancomprise a p-type composition having a larger band gap than thebarrier(s) located within the superlattice layer 12, which can result inan improved transparency of the electron blocking layer to radiationgenerated by the radiation generating structure 52. Furthermore, theelectron blocking layer 56 can comprise a graded composition, which canbe configured to decrease a resistance of the electron blocking layer56. For example, the electron blocking layer 56 can have a graded dopingthat increases or decreases by approximately 10⁴ cm⁻³, e.g., betweenapproximately 10¹⁶ and approximately 10²⁰ cm⁻³. Alternatively, theelectron blocking layer 56 can have a homogeneous doping within therange of approximately 10¹⁶ and approximately 10²⁰ cm⁻³.

The device structure 40 can include a contact 60. Contact 60 cancomprise any type of contact. In an embodiment, the contact 60 comprisesa p-type metal contact, such as a Schottky contact, a leaky Schottkycontact, a rectifying contact, and/or the like. In a more specificembodiment, the contact 60 at least partially reflects the radiationgenerated by the radiation generating structure 52 and can be formedfrom, among other things, aluminum, enhanced aluminum, aluminum siliconmonoxide, aluminum magnesium fluoride, rhodium, enhanced rhodium, gold,and/or the like. In another more specific embodiment, the contact 60 isat least partially transparent to the radiation generated by theradiation generating structure 52 and can be formed from, among otherthings, a metallic superlattice, in which each layer is at leastpartially transparent to the radiation. In either case, the contact 60can be directly adjacent to a transparent adhesion layer 58. Thetransparent adhesion layer 58 can be configured to improve ohmicproperties of the contact 60 and promote adhesion of the contact 60 to asurface of the semiconductor (e.g., layer 54). In an embodiment, thetransparent adhesion layer 58 is formed of nickel. However, it isunderstood that transparent adhesion layer 58 can be formed of anysuitable material, including Nickel oxyhydroxide (NiOx), Palladium (Pd),Molybdenum (Mo), Cobalt (Co), and/or the like.

The various layers in the device structure 40 can be formed using anytype of materials. In an embodiment, the device structure 40 comprises agroup III nitride-based heterostructure, in which one or more of thelayers 50, 56, 12, and 54 and radiation generating structure 52 areformed of various group III nitride materials using any solution.Additionally, contact 60 can be implemented without a transparentadhesion layer 58, and be formed of one or more layers of metal, such asfor example, one or more layers of titanium, aluminum, gold, chromium,nickel, platinum, lead, rhodium, and/or the like.

In an embodiment, one or more of the contacts 50, 54, 60 comprisesgraphene, which can be configured to be transparent to radiationgenerated by the radiation generating structure 52 and very conductive.For example, the p-type contact layer 54 to the superlattice layer 12and/or contact 60 can be at least partially formed of p-type graphene.Similarly, the n-type contact layer 50 can be at least partially formedof n-type graphene. In an embodiment, a contact 50, 54, 60 comprises agraphene composite contact, which includes a graphene sub-layer adjacentto a thin sub-layer of metal, which can improve current spreading in thecontact 50, 54, 60. In a further embodiment, the graphene compositecontact is at least partially transparent to the radiation generated bythe radiation generating structure 52. It is understood that the devicestructure 40 can include one or more layers, such as transparentadhesion layer 58 and/or contact 60, adjacent to a contact formed ofgraphene, such as contact 54, which are configured to improve lightextraction from the device structure 40, e.g., via a textured surface.

In an embodiment, a structure described herein can include one or morelayers having a composition selected such that the layer has atransparency of at least a target transparency to radiation, such asultraviolet radiation, of a target set of wavelengths. The layer cancomprise, for example, a p-type contact layer 54 (FIG. 6), an electronblocking layer 56 (FIG. 6), a superlattice layer 12 (FIG. 6), and/or thelike. For example, a layer can be a group III nitride-based layer, whichis composed of Al_(x)Ga_(1-x)N where the aluminum molar fraction (x) issufficiently high in some domains of the layer to result in the layerbeing at least partially transparent to ultraviolet radiation. In anembodiment, the layer can comprise a superlattice layer located in anemitting device configured to emit radiation having a dominantwavelength in the ultraviolet spectrum, and the composition of at leastone sub-layer in each period of the superlattice layer is configured tobe at least partially transparent to ultraviolet radiation having atarget wavelength corresponding to the ultraviolet radiation emitted bythe emitting device.

In an embodiment, the sub-layer has a thickness in a range betweenapproximately one and approximately one thousand nanometers.Furthermore, the sub-layer can have a graded doping that increases ordecreases by approximately 10⁴ cm⁻³, e.g., between approximately 10¹⁶and approximately 10²⁰ cm⁻³. Alternatively, the sub-layer can have ahomogeneous doping within the range of approximately 10¹⁶ andapproximately 10²⁰ cm⁻³. The doping can be any type of doping. Forexample, the doping can be: modulation doping; unintentional doping byimpurities from one or more of oxygen, hydrogen, and magnesium; adopant, such as magnesium and/or carbon, diffused from another dopedlayer or present in the growth chamber as residual elements; and/or thelike. In an embodiment, one or more sub-layers can be co-doped withmagnesium and carbon, where both the carbon and magnesium doping levelsare within the range of approximately 10¹⁶ and approximately 10²⁰ cm⁻³,but the combined concentration of the dopants does not exceedapproximately 10²⁰ cm⁻³. In another embodiment, the doping can alternatebetween two or more dopants. For example, a sub-layer can include carbondoping, while the adjacent sub-layer(s) can include magnesium doping.

An amount of transparency of a short period superlattice (SPSL) can beapproximated by computing the averaged band gap of the SPSL, anddeducing average absorption coefficient of the SPSL. The absorptioncoefficients depend on an absorption edge of the semiconductor material,which for materials formed of an AlGaN alloy, is a function of the molarfractions of the Al_(x)Ga_(1-x)N semiconductor alloy.

In an embodiment, the target transparency for the material is at leastten times more transparent than the least transparent layer of materialin the structure (e.g., GaN for a group III nitride-based device). Inthis case, an absorption coefficient of the semiconductor layer can beon the order of 10⁴ inverse centimeters or lower. In this case, a onemicron thick semiconductor layer will allow approximately thirty-sixpercent of the ultraviolet radiation to pass there through.

FIG. 7 shows a dependence of the absorption coefficient on thewavelength for various aluminum molar fractions (x) of anAl_(x)Ga_(1-x)N alloy according to an embodiment. In order to maintainan absorption coefficient of the semiconductor layer at orders of 10⁴inverse centimeters or lower, the content of aluminum in an SPSL barrierlayer can be chosen based on the corresponding target wavelength orrange of wavelengths. For example, for a target wavelength ofapproximately 250 nanometers, the aluminum molar fraction can beapproximately 0.7 or higher, whereas for a target wavelength ofapproximately 300 nanometers, the aluminum molar fraction can be as lowas approximately 0.4. FIG. 8 shows an illustrative chart for selectingan aluminum content of an Al_(x)Ga_(1-x)N alloy to maintain a targettransparency for a corresponding emitted wavelength, λ, according to anembodiment. In this case, the target transparency corresponds to anabsorption coefficient of the semiconductor layer on the order of 10⁴inverse centimeters. Note that in FIG. 8, the dependence of x=x(λ) islinear, with x=C˜λ+B, where C=−0.0048 nm⁻¹, and B=1.83.

In an embodiment, one or more sub-layers of the SPSL can have a gradedcomposition. For example, a sub-layer of the SPSL can be formed of anAl_(x)Ga_(1-x)N alloy, where the aluminum molar fraction, x, iscontinually varied in the vertical direction of the sub-layer.

In an embodiment, a device can include one or more layers with lateralregions configured to facilitate the transmission of radiation throughthe layer and lateral regions configured to facilitate current flowthrough the layer. For example, the layer can be a short periodsuperlattice, which includes barriers alternating with wells. In thiscase, the barriers can include both transparent regions, which areconfigured to reduce an amount of radiation that is absorbed in thelayer, and higher conductive regions, which are configured to keep thevoltage drop across the layer within a desired range. As used herein,the term lateral means the plane of the layer that is substantiallyparallel with the surface of the layer adjacent to another layer of thedevice. As described herein, the lateral cross section of the layer caninclude a set of transparent regions, which correspond to those regionshaving a relatively high aluminum content, and a set of higherconductive regions, which correspond to those regions having arelatively low aluminum content.

The set of transparent regions can be configured to allow a significantamount of the radiation to pass through the layer, while the set ofhigher conductive regions can be configured to keep the voltage dropacross the layer within a desired range (e.g., less than ten percent ofa total voltage drop across the structure). In an embodiment, the set oftransparent regions occupy at least ten percent of the lateral area ofthe layer, while the set of higher conductive regions occupy at leastapproximately two percent (five percent in a more specific embodiment)of the lateral area of the layer. Furthermore, in an embodiment, a bandgap of the higher conductive regions is at least five percent smallerthan the band gap of the transparent regions. In a more particularembodiment, the transparent regions comprise a transmission coefficientfor radiation of a target wavelength higher than approximately sixtypercent (eighty percent in a still more particular embodiment), whilethe higher conductive regions have a resistance per unit area tovertical current flow that is smaller than approximately 10⁻² ohm·cm².As used herein, the term transmission coefficient means the ratio of anamount of radiation exiting the region to an amount of radiationentering the region.

The transparent and conductive regions can be formed using any solution.For example, a layer can be grown using migration-enhanced metalorganicchemical vapor deposition (MEMOCVD). During the growth, inhomogeneitiesin the lateral direction of a molar fraction of one or more elements,such as aluminum, gallium, indium, boron, and/or the like, can beallowed in the layer. In an embodiment, such compositionalinhomogeneities can vary by at least one percent.

In an embodiment, a light emitting device structure can include one ormore structures configured to reduce an overall polarity of thestructure. In embodiment, the structure can form a cladding layer, ap-type contact layer, and/or the like, of a light emitting device. Inorder to confine a polarization charge within a sub-layer, the sub-layerthicknesses can be larger than a Bohr radius of the carriers. Using ap-type contact layer as an illustrative example, the Bohr radius, R_(B),can be calculated for hole carriers. In this case, the Bohr radius isgiven by R_(B)=4π∈²/m_(h)e², where ∈ is the permittivity of thematerial,  is the reduced Planck's constant, m_(h) is the hole restmass, and e is the elementary charge. For the compositionAl_(0.5)Ga_(0.5)N, the mass of an “average” hole is about four times theelectron rest mass (m_(h)˜4m_(e)), the permittivity is approximatelynine times the permittivity of free space (∈˜9∈₀), and the resultingBohr radius, R_(B), is approximately 9/4 of the Bohr radius of hydrogen,R_(H), that is R_(B)˜1.2 nm. A group III semiconductor layer having ahigher concentration of gallium will have a smaller hole mass (e.g., forGaN, m_(h)˜1.4). As a result, such a group III semiconductor layer canhave a Bohr radius, R_(B)˜6×R_(H)=3.2 nm.

An AlGaN film deposited by MOCVD on a substrate formed of sapphire, SiC,Si, and/or the like, typically grows with its gallium face up. Thisgrowth corresponds to the growth direction of the film being [0001], thepositive c-axis direction. However, growth of a heavily Mg-doped AlGaNlayer by MOCVD can produce a negative c-axis direction (N-face growth)of AlGaN. The inversion of polarity can reduce an overall “average”polarity within a given sub-layer. To this extent, FIG. 9 shows anillustrative lattice configuration of a gallium nitride layer includingdomain inversion according to an embodiment. As illustrated the layerincludes a plurality of lateral domains, at least one of which is anitride facing domain (N-face) and at least one of which is a galliumfacing domain (Ga-face). As illustrated, the polarization (P_(S)) andelectric field (E) vectors are inversed on either side of the boundarybetween the domains.

Structures 10, 30 described herein can be incorporated as part of, forexample, a transistor (e.g., a field effect transistor), aphotodetector, a monolithic and/or optoelectronic integrated circuit, ametal-semiconductor diode, a p-n junction diode, a switch, and/or thelike. In this case, the device can include one or more superlattices asbuffer layer(s), barrier layer(s), contact layer(s), and/or the like. Ina more particular embodiment, the periods of the superlattice layer areformed from AlInN.

While shown and described herein with respect to the fabrication of asuperlattice layer, it is understood that an embodiment of the inventioncan be applied to the fabrication of a heterostructure comprising a setof quantum wells and a set of barriers. The various sub-layers shown anddescribed herein can be formed using any solution. For example, thesuperlattice layers 12, 32 can be grown using a combination of metalloorganic chemical vapor deposition (MOCVD) and/or migration enhancedMOCVD (MEMOCVD), in which each period in the superlattice layer 12, 32requires at least two growth steps.

While shown and described herein as a method of designing and/orfabricating a structure and/or a corresponding semiconductor deviceincluding the structure, it is understood that aspects of the inventionfurther provide various alternative embodiments. For example, in oneembodiment, the invention provides a method of designing and/orfabricating a circuit that includes one or more of the semiconductordevices designed and fabricated as described herein (e.g., including oneor more superlattice layers 12, 32).

To this extent, FIG. 10 shows an illustrative flow diagram forfabricating a circuit 126 according to an embodiment. Initially, a usercan utilize a device design system 110 to generate a device design 112using a method described herein. The device design 112 can compriseprogram code, which can be used by a device fabrication system 114 togenerate a set of physical devices 116 according to the features definedby the device design 112. Similarly, the device design 112 can beprovided to a circuit design system 120 (e.g., as an available componentfor use in circuits), which a user can utilize to generate a circuitdesign 122 (e.g., by connecting one or more inputs and outputs tovarious devices included in a circuit). The circuit design 122 cancomprise program code that includes a device designed using a methoddescribed herein. In any event, the circuit design 122 and/or one ormore physical devices 116 can be provided to a circuit fabricationsystem 124, which can generate a physical circuit 126 according to thecircuit design 122. The physical circuit 126 can include one or moredevices 116 designed using a method described herein.

In another embodiment, the invention provides a device design system 110for designing and/or a device fabrication system 114 for fabricating asemiconductor device 116 by using a method described herein. In thiscase, the system 110, 114 can comprise a general purpose computingdevice, which is programmed to implement a method of designing and/orfabricating the semiconductor device 116 as described herein. Similarly,an embodiment of the invention provides a circuit design system 120 fordesigning and/or a circuit fabrication system 124 for fabricating acircuit 126 that includes at least one device 116 designed and/orfabricated using a method described herein. In this case, the system120, 124 can comprise a general purpose computing device, which isprogrammed to implement a method of designing and/or fabricating thecircuit 126 including at least one semiconductor device 116 as describedherein.

In still another embodiment, the invention provides a computer programfixed in at least one computer-readable medium, which when executed,enables a computer system to implement a method of designing and/orfabricating a semiconductor device as described herein. For example, thecomputer program can enable the device design system 110 to generate thedevice design 112 as described herein. To this extent, thecomputer-readable medium includes program code, which implements some orall of a process described herein when executed by the computer system.It is understood that the term “computer-readable medium” comprises oneor more of any type of tangible medium of expression, now known or laterdeveloped, from which a copy of the program code can be perceived,reproduced, or otherwise communicated by a computing device. Forexample, the computer-readable medium can comprise: one or more portablestorage articles of manufacture; one or more memory/storage componentsof a computing device; paper; and/or the like.

In another embodiment, the invention provides a method of providing acopy of program code, which implements some or all of a processdescribed herein when executed by a computer system. In this case, acomputer system can process a copy of the program code to generate andtransmit, for reception at a second, distinct location, a set of datasignals that has one or more of its characteristics set and/or changedin such a manner as to encode a copy of the program code in the set ofdata signals. Similarly, an embodiment of the invention provides amethod of acquiring a copy of program code that implements some or allof a process described herein, which includes a computer systemreceiving the set of data signals described herein, and translating theset of data signals into a copy of the computer program fixed in atleast one computer-readable medium. In either case, the set of datasignals can be transmitted/received using any type of communicationslink.

In still another embodiment, the invention provides a method ofgenerating a device design system 110 for designing and/or a devicefabrication system 114 for fabricating a semiconductor device asdescribed herein. In this case, a computer system can be obtained (e.g.,created, maintained, made available, etc.) and one or more componentsfor performing a process described herein can be obtained (e.g.,created, purchased, used, modified, etc.) and deployed to the computersystem. To this extent, the deployment can comprise one or more of: (1)installing program code on a computing device; (2) adding one or morecomputing and/or I/O devices to the computer system; (3) incorporatingand/or modifying the computer system to enable it to perform a processdescribed herein; and/or the like.

The foregoing description of various aspects of the invention has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and obviously, many modifications and variations arepossible. Such modifications and variations that may be apparent to anindividual in the art are included within the scope of the invention asdefined by the accompanying claims.

What is claimed is:
 1. A structure comprising: a first layer; and asuperlattice layer having a first side adjacent to the first layer, thesuperlattice layer including a plurality of periods, each of theplurality of periods including: a first sub-layer having a first groupIII nitride composition and a first polarization, wherein the firstgroup III nitride composition is selected such that the first sub-layerhas a transparency of at least a target transparency to ultravioletradiation of a target wavelength; and a second sub-layer adjacent to thefirst sub-layer, the second sub-layer having a second group III nitridecomposition distinct from the first group III nitride composition and asecond polarization opposite the first polarization.
 2. The structure ofclaim 1, wherein the first group III nitride composition includesaluminum, and wherein a molar fraction of aluminum in the first groupIII nitride composition is selected based on the target transparency toultraviolet radiation of the target wavelength.
 3. The structure ofclaim 2, wherein the first group III nitride composition is anAl_(x)Ga_(1-x)N alloy.
 4. The structure of claim 3, wherein the firstgroup III nitride composition includes an aluminum molar fraction xselected using the formula C·λ+B, where C=−0.0048 nm⁻¹, B=1.83, and λ isthe target wavelength of the ultraviolet radiation.
 5. The structure ofclaim 1, wherein a composition of the first sub-layer varies alonglateral dimensions of the first sub-layer such that a lateral crosssection of the first sub-layer includes: a set of transparent regions,each transparent region having a transmission coefficient for the targetwavelength greater than or equal to approximately sixty percent, whereinthe set of transparent regions are at least ten percent of an area ofthe lateral cross section of the first sub-layer; and a set of higherconductive regions occupying a sufficient area of the area of thelateral cross section of the first sub-layer and having an averageresistance per unit area to a vertical current flow resulting in a totalvoltage drop across the superlattice layer of less than ten percent of atotal voltage drop across the structure.
 6. The structure of claim 5,wherein each of the set of higher conductive regions comprises a molarfraction of aluminum providing a band gap at least five percent smallerthan a band gap of the set of transparent regions.
 7. The structure ofclaim 1, wherein the first layer comprises an electron blocking layer,the structure further comprising a multiple quantum well structureadjacent to a second side of the superlattice layer, wherein the secondside is opposite the first side.
 8. The structure of claim 1, whereinthe first layer and the superlattice layer form at least one of: acladding layer or a p-type contact.
 9. The structure of claim 1, whereinthe ultraviolet radiation is in a range including a peak emissionwavelength of ultraviolet radiation emitted by the structure.
 10. Thestructure of claim 1, wherein a thickness of the first sub-layer isselected to be at least a size of a Bohr radius for hole carriers. 11.The structure of claim 1, wherein the first sub-layer includes a firstlateral region and a second lateral region, wherein a polarization ofthe first lateral region is opposite a polarization of the secondlateral region due to epitaxial growth with face inversion.
 12. Thestructure of claim 11, wherein the first and second lateral regionsoccupy a combined planar area of at least two percent of a total planararea of the sub-layer.
 13. The structure of claim 1, wherein thestructure comprises a heterostructure device configured to operate as atleast one of: a light emitting diode, a superluminescent diode, or alaser.
 14. A method comprising: creating a structure design for adevice, the structure design including a first layer and a superlatticelayer having a first side adjacent to the first layer, the superlatticelayer comprising a plurality of periods, the creating the structuredesign including: selecting a first group III nitride composition havinga first polarization for a first sub-layer of each of the plurality ofperiods, wherein the first group III nitride composition is selectedsuch that the first sub-layer has a transparency of at least a targettransparency to ultraviolet radiation of a target wavelength; andselecting a second group III nitride composition having a secondpolarization for a second sub-layer of each of the plurality of periods,wherein the second sub-layer is adjacent to the first sub-layer, andwherein the second group III nitride composition is distinct from thefirst group III nitride composition and the second polarization isopposite the first polarization.
 15. The method of claim 14, wherein thefirst group III nitride composition includes aluminum, and wherein theselecting includes selecting a molar fraction of aluminum in the firstgroup III nitride composition based on the target transparency toultraviolet radiation of the target wavelength.
 16. The method of claim15, wherein the first group III nitride composition is anAl_(x)Ga_(1-x)N alloy, and wherein the first group III nitridecomposition includes an aluminum molar fraction x selected using theformula C·λ+B, where C=−0.0048 nm⁻¹, B=1.83, and λ is the targetwavelength of the ultraviolet radiation.
 17. The method of claim 14,further comprising fabricating at least one device according to thestructure design.
 18. The method of claim 17, wherein the fabricatingincludes growing the first sub-layer such that a composition of thefirst sub-layer varies along lateral dimensions of the first sub-layersuch that a lateral cross section of the first sub-layer includes: a setof transparent regions, each transparent region having a transmissioncoefficient for the target wavelength greater than or equal toapproximately sixty percent, wherein the set of transparent regions areat least ten percent of an area of the lateral cross section of thefirst sub-layer; and a set of higher conductive regions occupying asufficient area of the area of the lateral cross section of the firstsub-layer and having an average resistance per unit area to a verticalcurrent flow resulting in a total voltage drop across the superlatticelayer of less than ten percent of a total voltage drop across thestructure.
 19. The method of claim 18, wherein each of the set of higherconductive regions comprises a molar fraction of aluminum providing aband gap at least five percent smaller than a band gap of the set oftransparent regions.
 20. A group III nitride-based device comprising: ap-type contact comprising: a first p-type layer; and a p-typesuperlattice layer including a plurality of periods, each of theplurality of periods including: a first sub-layer having a first groupIII nitride-based composition and a first polarization, wherein thefirst group III nitride-based composition is selected such that thefirst sub-layer has a transparency of at least a target transparency toultraviolet radiation of a target wavelength; and a second sub-layeradjacent to the first sub-layer, the second sub-layer having a secondgroup III nitride-based composition distinct from the first compositionand a second polarization opposite the first polarization, wherein thefirst polarization and the second polarization comprise at least one of:a strain-induced polarization or a spontaneous polarization.